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Home , Explosive lens, Lillian Hoddeson, Perseus (spy), Rudolf Peierls, Seth Neddermeyer

Submission for ANS NT issue, LA-UR-20-27638 version 3

Who Invented the Nuclear Test’s Christy Gadget? Patents and Evidence from the Archives

Thomas A. Chadwick* and M.B. Chadwick Los Alamos National Laboratory Los Alamos, NM 87545 Abstract: The Christy Gadget is the informal name for the device detonated in the Trinity test on July 16, 1945. In September 1944, Robert Christy, working in the theoretical implosion group, proposed a novel concept that altered the design of the nuclear core in . While scientists originally intended to use a hollow sphere of plutonium, this design entailed substantial risk, due to the likelihood of asymmetries resulting from implosion. Christy proposed changing the design to a sphere of plutonium with a modulated neutron source, and the design was eventually adopted, tested at Trinity, and used in the attack on . While there is no question regarding the important role that Christy played in demonstrating its feasibility as a reliable design, there is a debate as to who initially proposed the idea; though most sources have attributed this invention to Christy, some historical sources have attributed credit to Christy’s group leader, , or indeed other scientists. This paper seeks to outline and resolve this dispute. We present new unclassified evidence extracted from previously unavailable sources (to unclassified audiences) from the Research Center archives at Los Alamos National Laboratory. This evidence consists of 1945–1946 patent documentation, oral history interview tapes of Christy and Peierls, and monthly 1944 progress reports from the Theoretical Division. Though Christy and Peierls share joint credit on the patent, both Christy’s and Peierls’ words and writings, together with sources from and , support the traditional view that Christy was indeed the originator of the idea. While Christy does deserve the majority of the credit for the invention and design, we acknowledge the important role Peierls and von Neumann played in its development.

1. Introduction implosion, including jetting, spalling, and Rayleigh- The Trinity test, performed on July 16, 1945, was the first Taylor instabilities. Scientists feared that these detonation of an atomic weapon. The test was a asymmetries might prevent the reliable assembly of a remarkable achievement: it was an amalgamation of . numerous scientific discoveries and inventions and it Consequently, many scientists remained skeptical brought the war-torn world into a new atomic age. about the feasibility of successfully building an implosion Fundamentally, it proved the feasibility of a plutonium device on the urgent timeframe needed to impact the war implosion design that would be subsequently used in the effort. Segrè’s discovery that reactor-produced plutonium Fat Man device detonated over Nagasaki. Robert Christy, had too-high a spontaneous neutron emission, preventing a scientist working in the Theoretical Division during the a gun-type assembly design, and the challenges , has his name firmly tied to the Trinity of producing large quantities of , test. The gadget tested in Trinity is informally known as elevated the implosion design to the center of the the “Christy Gadget” because Christy made an important, laboratory mission. Oppenheimer reorganized the novel proposal in September 1944 for a new design of the laboratory in August 1944 to orient the laboratory mission plutonium core. around the implosion device, establishing two divisions, Until late 1944, scientists at the Manhattan Project the Weapons or Gadget Division (G) and the hoped to be able to use a hollow plutonium shell as the Division (X), both intended to primarily focus core of the implosion device. Upon detonation, the high on the implosion design.1,2 As a sign of the urgency, when would create an imploding shock wave that visited the laboratory on August 1, 1944, would compress the hollow plutonium shell into a super- he gave a colloquium and advised the audience that they critical mass, creating a nuclear yield. While this design had already used “half of the time of the estimated was elegant and efficient, in practice, after extensive maximum to produce a successful gadget.”3 experimentation it was still plagued by asymmetries upon

* [email protected]

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In September 1944, Christy proposed a new While some sources attribute credit for the invention to conservative design for the plutonium , suggesting a scientists other than Christy, the evidence for these claims change from the hollow plutonium shell to a solid sphere is weak. As we shall show, the references given for of plutonium with a modulated neutron source. The solid Arnold’s claim do not support the view that Peierls was sphere of plutonium was surrounded by a tamper and the original inventor. Furthermore, the idea that Teller or explosives that would simultaneously explode to form a von Neumann was the principal originator of the Christy convergent detonation wave, compressing the plutonium Gadget design is not supported by evidence. Indeed, none symmetrically. The modulated neutron source refers to of these remarkable scientists actually made this claim the concept that a source was desired that provided very themselves. While we recognize that this invention few neutrons at the beginning of the implosion (to avoid required a collaborative effort, we conclude that the name pre-initiation) but a large number of neutrons later when of this invention is aptly bestowed and argue that Robert the plutonium would be compressed. This design relied Christy deserves principal credit for this invention. on assembling a near-critical mass of plutonium before compressing it, increasing its density and creating a 2. Dramatis Personae supercritical assembly. While Christy’s new invention was thought to be less efficient than the hollow shell, the A. Robert Christy solid sphere was a more reliable design because it Robert Christy (Fig. 1), a Canadian who was naturalized minimized the implosion asymmetries. Although there is as a US citizen, did his PhD work under J. Robert no question that Christy performed important Oppenheimer at the University of California, Berkeley, hydrodynamic and nuclear research to prove the before moving to the University of to work on feasibility of this design, there is a debate in the open the reactor experiments with Fermi and Wigner. He was literature as to who invented the concept. As will be an early recruit to the Manhattan Project, arriving in Los discussed further, the 1946 patent is in the names of both Alamos in 1943. Christy and Peierls. While a number of sources make Christy helped work on the water boiler reactor and claims about the origins of this idea, we have yet to find then worked in the Theoretical Division’s Hydro- any source which summarizes the evidence into an dynamics of Implosion group (T-1), first under Edward argument about the genesis of the Christy Gadget. The Teller and then under Rudolf Peierls. In June 1944, purpose of this paper is to review evidence from both Oppenheimer and the Theoretical Division Leader Hans open and classified sources, and to provide unclassified Bethe transferred Teller from the T-1 leadership role to extracts from the National Security Research Center instead focus on thermonuclear concepts (the “Super”). In 6 (NSRC) at Los Alamos to answer this question. the 1986 oral history interview with Hoddeson (see Sec. While Christy’s name has become synonymous with 4), Christy couldn’t remember that Teller was his group this novel design, many different sources attribute credit leader, stating, “I don’t remember working under Teller” for this invention to scientists other than Robert Christy. and “Mostly he pursued his own thing. He wasn’t much † In Britain and the H-Bomb, British historian Lorna of a team player”! Teller himself noted that he wasn’t Arnold claims that the solid sphere implosion design was the right person to lead the calculational implosion effort, actually invented by Rudolf Peierls, the Hydrodynamics and that Bethe’s initial desire for Teller to lead this of Implosion group leader and a member of the British needed a different solution. When the British scientists Mission to Los Alamos.4 Arnold’s claim is referenced by started arriving in Los Alamos, Teller said, “Bethe saw , Frank Close, in his excellent 2019 book Peierls as ready-made for the task he had in mind for me. Trinity: The Treachery and Pursuit of the Most Peierls could tackle the calculations of implosion. So Dangerous Spy in History. According to Close, an Peierls, with the help of a small group, diligently began 7 “authoritative history” credits Peierls with the invention.5 the Herculean labor.” Even figures like Edward Teller, one of the key inventors After Los Alamos, Christy had a scientific research and of the hydrogen bomb, and , the teaching career at Caltech and served as President of Hungarian mathematician who made numerous Caltech for a period. innovations on implosion and high- B. Rudolf Peierls systems, have been credited with inventing the solid- sphere implosion design. Rudolf Peierls (Fig. 2) was a German-born physicist known for his expertise in , material

† Following the Oppenheimer security hearing in the 1950s, Christy had cold relations with Teller. Teller’s memoirs describe Christy spurning him (Ref. 7).

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science, and shock hydrodynamics. In Germany, he C. John von Neumann studied physics under Heisenberg and Pauli before An émigré from Hungary to Princeton, the brilliant moving to the in Cambridge. He mathematician and theoretical physicist came to Los became a naturalized British citizen and after the war was Alamos from late 1943 onwards as a consultant, at knighted. Peierls was a coauthor of the famous March Oppenheimer’s invitation. 1940 Frisch-Peierls memorandum. This in turn led to the Von Neumann (Fig. 3) was a renowned expert in shock creation of the and its report,8 drafted hydrodynamics and pioneered the development of by Sir , which expedited research in computing on electronic machines. Stan Ulam’s obituary atomic weapons in Britain and in the USA in the early of von Neumann14 spoke of his “ability, perhaps some- . Peierls came to Los Alamos in 1944 as part of the what rare among mathematicians, to commune with the British Mission, becoming leader of the Theoretical , understand their language, and to transform it Division’s T-1 group. almost instantly into a mathematician's schemes and It is perhaps useful to provide some background that expressions. Then, after following the problems as such, illuminates Peierls’ brilliance and the extent of his he could translate them back into expressions in common essential contribution to the Manhattan project. use among physicists.” Von Neumann suggested using In 1942, Oppenheimer was already communicating large amounts of high explosives to cause exceedingly with Peierls by letter on detailed questions in nuclear high velocities in an implosion, and both von Neumann science and weapons physics.9 As part of his goal to and Teller had the insight that this would increase recruit Peierls to Los Alamos, Oppenheimer wrote a letter compression to create a critical mass (Ref. 7, p. 175). to on February 14, 1944, in which he Together with James Tuck (who had been working on praised Peierls’ expertise in the hydrodynamics of related shaped-charge research in England15) and Seth implosion.10 Oppenheimer noted that the British had Neddermeyer, he suggested that a symmetric implosion some complementary technical knowledge of blast waves could be accomplished using a high-explosive lens that was needed in Los Alamos11. system. On July 18, 1949, Bethe wrote a letter to Carroll L. Wilson, general manager of the Atomic Energy D. Ralph Carlisle Smith Commission, documenting the role of various British Major Ralph Carlisle Smith (Fig. 4) came to Los Alamos scientists. Of Peierls, he said, “He joined the Los Alamos in 1944. He was an Office of Scientific Research and Project at a time when it was impossible to find a Development (OSRD) representative tasked by competent senior theoretical physicist to head the group Oppenheimer to manage patents during the Manhattan without crippling some other phases of the work. Peierls Project. He subsequently became an assistant director of was, in my opinion, the most effective group leader in the the Laboratory from 1947 to 1957. He later moved to Theoretical Division. . . . He directed the entire difficult become the president of Highlands work on implosion hydrodynamics, contributing a very University. He was a patent attorney with a first degree in great fraction of it himself. . . . Due to the work of the chemical engineering from the Rensselaer Polytechnic group the project had sufficient theoretical knowledge of Institute. the implosion to feel confident of the feasibility of the An organizational chart of the Theoretical Division, implosion weapon.” Director , in a letter May 10, 1945, is shown in Fig. 5, showing Peierls and on the same date also to Wilson, stated, “It should be Christy in T-1 as well as listing numerous other noted that the British Mission supplied the major portion luminaries. (Note that the Los Alamos WRL journal’s of experience in the field of theoretical hydro- 2021 special issue on Trinity has a paper by Shlachter dynamics . . . the U.S. was largely lacking in personnel describing the contributions of the Jewish members of T- experienced in this field of classical physics.”12 Division.) Frank Close’s book refers to Peierls as the father of the atomic bomb, a title that is traditionally also applied to J. 3. Discrepancies Over the Inventor of the Christy Robert Oppenheimer. This obtains from his joint Gadget authorship on the Christy gadget patent and his shock , in her book Britain and the H-Bomb, hydrodynamic calculation leadership at Los Alamos. It asserts that Peierls came up with the idea of the Christy also reflects his early 1939 Cambridge Philosophical Gadget (Ref. 4, p. 254). While Arnold’s book mainly Society paper that derives a critical mass formula for examines the British efforts to build a hydrogen bomb at unmoderated fast-neutron systems and the Frisch-Peierls Aldermaston in the 1950s, she also discusses the 1940 memorandum that first computed the critical mass contributions of the British Mission at Los Alamos. In a and efficiency of an enriched uranium bomb.13 footnote, Arnold claims that the “Christy Gadget” was

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actually Peierls’ idea (Ref. 4, p. 254). It is worth quoting named on the patent, see Sec. 4A). Smith also notes that Arnold in full: Peierls was an expert in hydrodynamics and that his work was crucial for the development of the implosion device, “At Los Alamos, Peierls and Fuchs provided two-thirds contributing to “all phases of weapon development, of the team which made the implosion development including implosion and Super.”18 While this document possible and contributed to all phases of weapons does give Peierls a role in the invention of the Christy development (including the Super). The solid implosion Gadget and commends his expertise in hydrodynamics, gadget invented by Peierls and Christy is commonly no statement in Smith’s summary corroborates the claim called the Christy gadget but was Peierls’ idea. Tuck, that Peierls originated the idea.§ independently and with the US scientists Neddermeyer and von Neumann, suggested the lens system for A. Invention Claims Attributed to John von Neumann implosion and worked with Bethe on the initiator. Frisch and Edward Teller made many contributions, especially to critical mass Smith’s memorandum on the contributions of the assembly studies. Bretscher made considerable British Mission also gives some credit to John von contributions to Super feasibility studies. Titterton did Neumann for first developing the idea of the solid-sphere outstanding work, particularly on electronic circuit design. He ascribes the “Christy Gadget” invention developments. Rotblat worked with several others in the jointly to Peierls and Christy, but also states that “Solid field of experimental nuclear physics. See F. Szasz, implosion had been suggested by von Neumann in his British Scientists and the Manhattan Project, pp. 148– early patent application but the idea of a modulated 51.” neutron source with the solid implosion was that of Peierls and Christy.” It should be noted that Arnold’s citation is to a book It is true that the Christy-Peierls Patent S-3956X gives entitled British Scientists and the Manhattan Project by “prior art” as von Neumann’s 1943 patent S-673X (Fig. Szasz. However, the information in Szasz’s book is 6). The key contribution of von Neumann’s patent was to actually the appendix that reproduces a memorandum by propose using a large mass of high explosive to cause Ralph Carlisle Smith, a memorandum that is discussed plutonium compression. Von Neumann discusses below. It represents a fair reproduction, except for the numerous concepts that include a hollow shell of words “but was Peierls’ idea.”‡ Close’s new book Trinity: plutonium (later called the “standard gadget”) as well as The Treachery and Pursuit of the Most Dangerous Spy in designs with a solid spherical plutonium component. History cites Arnold’s claim (Ref. 5 p. 130 and his However, his patent does not include the eventual solid- Reference 13, p. 451). sphere Christy gadget design per se. When von Neumann Arnold’s claim contradicts other sources, most notably discussed a solid component, the design configuration Critical Assembly, a book by several Los Alamos differed from the Christy Gadget design, in that it historians,16 and deserve some attention. She cites only included an air gap and more complex components in one reference to support the claim, the 1949 addition to its “levitated” solid sphere. While these memorandum written by Ralph Carlisle Smith.17 The designs were used in later Los Alamos tests after the war, evidence from LANL’s patent collection indicates that they lacked the simplicity and robustness of the Christy- Smith was intimately involved with the patent creation Peierls invention. Von Neumann’s design was one of the process, as many of the drafts and administrative papers many concepts proposed during the Manhattan Project, bear his signature. At the conclusion of the project, Smith but it is inaccurate to label these design ideas as wrote a summary of the contributions of the British synonymous with the Christy Gadget. Mission, with a concise description of each of their Thus, our assessment from reading von Neumann’s contributions to the project. It is clear that he takes great patent is that Ralph Carlisle Smith’s explanation was not care to communicate fairly the credit due to the various quite right. He slightly mischaracterized the essential scientists. novel idea of Peierls and Christy as a modulated source But Smith’s summary in no way corroborates Arnold’s (the neutron initiator) with solid sphere, instead of also claim. Smith writes that Peierls and Christy were “joint emphasizing the lack of free space within the explosives. inventors” of the Christy Gadget, providing both with It has also been suggested that Edward Teller invented credit for the invention (as is appropriate since both are the idea of a solid-sphere implosion device. Alex

‡ Arnold’s statement of the British contribution to implosion being “two- § We think that Smith wrote “Peierls” before “Christy” simply because thirds” (the third being von Neumann or Christy, presumably) is from his entire paragraph was focused on Peierls’ role in the Manhattan Smith’s memorandum. Project, and not because he thought Peierls originated the idea.

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Wellerstein,19 a history of science professor at the Stevens parts of the patent documentation still cannot be released, Institute of Technology, states: we have been able to make some part of the patent openly available here, and it remains one of the most exciting “The solid-core concept was originally proposed pieces of new evidence on this topic. In particular, Fig. 7 by Edward Teller, whose experience working with provides one example of the claim that appears in the George Gamow on the iron core of the Earth gave him patent. The paragraph begins with “It is claimed”, and the somewhat unintuitive knowledge that even very dense then proceed to lay out the characteristics of the solid- materials can be compressed to even higher densities sphere implosion device. under many megabars of uniform . Christy, for his part, was the guy who took it from the ‘Teller’s interesting The patent describes the implosion design of a solid but potentially wrong idea’ phase to the ‘so will it plutonium sphere (without an air gap) with a modulated actually work?’ phase. And he did a good job of that— neutron source. The modulated neutron source was everyone was convinced that a solid core bomb would be both plausible and easier than the alternatives (such as a intended to selectively supply a large number of neutrons hollow core bomb).” at the time of compression to initiate a . Alternative external initiator concepts were However, although Teller’s Memoirs (Ref. 7, p. 175, discussed for shell implosions before the Christy Gadget see Sec. 5B) does describe the increased compression that proposal. The idea of a modulated neutron source at the occurs under pressure and these memoirs provide the center of the gadget was presented in the early 1945 analogy of the Earth’s iron core, this is given in the Christy-only version of the patent, but R.C. Smith’s context of the original “standard gadget” shell implosion previously quoted statement, “but the idea of a modulated and not in the solid ball design. Our reports in the NSRC neutron source with the solid implosion was that of do show a Teller patent (S1201X, Nov. 1, 1945, Box 3) Peierls and Christy,” suggests that both Christy and that includes a levitated solid sphere, but like von Peierls both played a role in this aspect of the invention. Neumann’s 1943 patent, it also includes an air gap. In For each individual patent, the NSRC maintains fact, Teller’s own words, reproduced below in Sec. 5B, handwritten and typed rough drafts, in addition to the clearly show that he credits Christy with the “ingenious” final version of the patent, all of which provide a glimpse solid ball invention. at the process behind creating the patent. These sets of documents provide strong evidence that Christy should be 4. Sources from Los Alamos NSRC recognized as the originator of the invention. Christy wrote out the first draft in 1945, with just his name on the A. Patent Collections document as sole inventor. Figure 8, the earlier Christy- During the Manhattan Project, scientists were only 1945 version, shows the invention to have occurred encouraged to document their innovations in patent “prior to October 2, 1944,” and cites Christy’s notebook applications, with the assistance of Ralph Carlisle Smith. A147 documenting the invention. Professor Sabine Lee noted that “Political leaders . . . After the war, this version was given to Peierls for his had been discussing the question of intellectual review. Peierls made a number of small revisions by hand ownership and commercial rights throughout the war, to the typed document and wrote “and Peierls” next to and they considered the implications of individual Christy’s name (Fig. 9). The patent documents detail the countries’ contributions to the international nuclear transition from Christy’s hand-written draft to the 20 venture as significant in view of post-war bargaining.” subsequent version with Peierls’ name added. Then, the These patents (final versions and early drafts), together final document was prepared with both Christy’s and with extensive related documentation and correspond- Peierls’ names, which appeared on the final patent ence, exist in the NSRC. There are numerous patents submitted to R.C. Smith.21** related to implosion concepts in the names of well-known A number of possible conclusions can be drawn by re- scientists such as von Neumann, Bethe, Neddermeyer, creating the history of the patent. The most likely scenario Teller, Fuchs, as well as Christy and Peierls, dating from is that Christy originated the idea and wrote the first draft, 1943 on. Robert Christy and Rudolf Peierls jointly filed a but sometime after this, discussions occurred between patent in January 1946 entitled “Method and Apparatus Christy, Peierls, and possibly Smith and others, leading to for Explosively Releasing Nuclear Energy,” which the view that the fairest course of action was to add patented the idea of the “Christy Gadget.” While some

** The final Christy-Peierls 1946 patent lists Peierls as a subject of the King of Great Britain and they were paid one dollar for transferring their rights and title of this invention to the US Government.

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Peierls’ name to recognize his contributions. This beautiful technique just because of the practical hypothesis is expanded upon in our conclusions. There is problems. So I was then pleased to devise a proposal that also an indication in Smith’s 1946 letter to Christy that convinced even the skeptics that the implosion could discussions with government representatives in work . . . and that’s the way I viewed it, as a, as kind of Washington also played a role in adding Peierls (Fig. 10). the worst implosion you could design, one without any Some additional images from the patent records are hole in the middle, but that would convince the doubters given in our report22. that they could go ahead with implosion.”

B. Oral History Tapes Hoddeson: “Do you remember the responses initially to your suggestion?” Christy Interview In 1986, LANL historian conducted an Christy: “Well I remember talking to various people interview with Robert Christy, in which he describes his including Oppenheimer and I think the responses were role in the invention and development of the implosion favorable, namely that it looked as though you could device. He comes across as transparent, and not boastful, hardly develop asymmetries in a solid sphere and the real when he repeatedly uses the first person singular—“I” not question was whether you could—there was no place for them to go—and so the real question was whether or not “we”—in describing his insights and calculations. the implosion gave you a sufficient compression so that In his remarks, Christy remembers putting forward the you would get a real significant yield, and I had made proposal for the Christy Gadget and calculated the calculations based on some of the approximate efficiency of the solid-sphere design, finding it to be calculations I did that indicated it would, and then there “reasonably efficient.” Christy’s proposal assuaged were various measurements made by RaLa and X-Ray skeptical colleagues who doubted the feasibility of the methods and so forth, and again, these also confirmed implosion design. that there was a real compression, that looked as though He said, “I do remember putting forward the proposal it would work. So I think things fell in line pretty well of an implosion of a solid. . . . I don’t know where the idea because the various experimental tests agreed pretty well came from, I just did it.” Here, he is ruminating on the with the calculations and it looked as though it would mysteries of how the creation idea originated in his own really work.” mind. (And in fact, as described in Sec. 4D, in the patent Christy does describe how this idea originated in his Peierls Interview thinking.) Below, we quote verbatim unclassified text Lillian Hoddeson also conducted an interview with from that interview: Rudolf Peierls that same year.23 When Hoddeson asks Peierls, “Had Christy not come up with the solid sphere Christy: “I was a believer in implosion simply because idea . . . do you think that there would have ultimately from the point of view of theory it was ideal, it was been a successful shell implosion device given what you absolutely perfect. The only difficulty was, it wasn’t sure know about the results?” her phrasing of the question you could make it go. But that didn’t bother me as a clearly ascribes credit to Christy for the invention of the theorist. From the theoretical point of view it had every Christy Gadget design. Peierls’ response is telling: advantage.” instead of contradicting her, he simply answers the . . . question. Hoddeson then asks a question regarding the Christy: “I don’t remember when the name ‘Christy innovative of the Christy Gadget. Once again, Gadget’ was first attached to it. I do remember putting Hoddeson’s use of language is telling. She asks, “Also, forward the proposal of an implosion of a solid. . . . I had Christy not come up with the idea of a solid . . . ?” don’t know where the idea came from except that there clearly attributing credit to Christy. Once again, Peierls were all these worries about what happens at the inside answers the question without contradiction. Peierls’ tacit surface, spalling, jetting and so forth, and so at one acceptance of Hoddeson’s questions provides evidence point—I don’t know exactly what preceded it—I tried to that he does not consider himself to be the principal do it for a solid sphere, to see whether that would be a inventor of the gadget, and towards the end, he states, possibility, and it seemed as though it was. That is, it gave “Now, Christy’s idea was to . . .” Again, we quote a reasonable efficiency. But I don’t remember where the verbatim unclassified extracts from that interview, where idea came from, I just did it. And, I was always a strong Peierls discusses Bethe’s Theoretical Division progress proponent of the implosion method and I guess I was reports from December 1944 and January 1945: always worried at all of the troubles that there were, and all of the asymmetries, and jetting, and spalling and Peierls: “This is the first time that numerical work on the that . . . . It worried me that people might discard this IBM machines has been done for the Christy Gadget. So,

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it took that long for this to be taken that seriously, because all the other numerical calculations still related to the Hoddeson: “Had Christy not come up with the solid hollow shell.” sphere idea . . . do you think that there would have ultimately been a successful shell implosion device given Hoddeson: “Why do you think it took so long?” what you know about the results?”

Peierls: “Why it took so long to adapt the IBM Peierls: “Well there were sufficient doubts. I mean, calculations to the Christy gadget? Well, I think the obviously one would have continued exploring all these answer must have been that it took a long time before the questions of instabilities and spalling and jets and so on, decision was reached that the Christy Gadget was the and probably one could have reduced the uncertainties main thing to aim at.” sufficiently, but one would be, in the final Trinity type test, one would have been less confident than with a solid Hoddeson: “I don’t think that decision was taken till gadget.” February.” Hoddeson: “Also, had Christy not come up with the idea Peierls: “Obviously before taking that decision, you of a solid . . . was it an obvious enough step then to take would have wanted to see the results of the IBM to go in that direction. . . . How much of an innovation calculations. But until then it was probably not a was that suggestion? Would it probably have been sufficiently serious candidate. And also, there was something that eventually one of the other people working probably the point that they had started the calculations might have come to?” on the other gadget and obviously they should be finished before you changed horses.” Peierls: “It is very hard to say. I mean it did certainly come as a surprise. To say well, why do we have to have Hoddeson (quoting Bethe’s summary): “Encouraged by a hollow shell? Why can’t we just . . . . You see, there is a the RaLa results, we have felt it desirable to make gradual transition. The first purpose of the implosion idea immediately an investigation of a solid-sphere implosion of a hollow shell was simply a very fast way of assembling by an IBM computer.” the material from the subcritical into a critical stage and there, the compression, the idea of compression, didn’t Peierls (on his report from the end of December 1944 that play a large part. Then one noticed in the course of doing became LAMS-182 on the problem of asymmetries in that, that one could gain considerably in compression, implosion, not for the solid sphere, but for the standard and therefore in density and therefore in criticality. But implosion): “This is still relating to the hollow shell . . . one thought of this increasing density as a result of all the Tuck’s X-ray shots are mentioned, which are particularly material collapsing together, and therefore its kinetic good at showing up jets, and Koski’s. But in general, it energy being converted to potential energy. . . . So, you seems first of all that it is possible to avoid jets by using had three stages of the explosion of the detonation wave lenses, and once you have no jets the collapse of a being converted to the kinetic energy of the shell and then spherical shell should not be too unstable. So, this paints as the collapse, the kinetic energy turning into potential a fairly optimistic picture, but with various energy and therefore compression. Now, Christy’s idea reservations. . . . But still, leaving various problems to be was to notice that you could leave out of the intermediate solved practically, like getting lenses that are sufficiently stage and convert the energy of the detonation wave well designed and sufficiently uniform and so on.” directly into potential energy. That was a very novel thought. Now, how likely it is that someone else would Peierls: “This report comes to a very serious have come up with that, I just don’t know.” recommendation of the solid gadget, in January. . . . There is an interesting remark about why it was relatively Christy’s invention is certainly novel, and worthy of a late—the IBM calculations turned over to the Christy patent! The interview above provides clear evidence that gadget . . . one doesn’t appreciate today how slow these Peierls viewed Christy as the principal originator of the calculations were. . . . In January the IBM machine solid-sphere invention. completed the 7th problem that they had done since they started, which was in April. The solid gadget was problem C. Theoretical Division Monthly Progress Reports number 8. . . . Each calculation is almost a month. Perhaps the first reference to the solid Gadget concept, Bethe’s progress report then mentions that the decision beyond Christy’s notebook, was given in the Theoretical was frozen: the Christy gadget with initiator. The Division’s September 1944 monthly progress report decision was taken during the month.” LAMS-14924, see Fig. 11. Bethe introduces this new work

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“as a possible further insurance against failure to have Division progress Report, 31. Sept. 1944, LAMS-149, pp symmetry in an implosion of more standard design.” 2-4, 11. According to the patent application, Christy’s Subsequently, each group leader provided a summary of conception took place prior to 2 Oct. 1944. The progress in their group. Peierls writes for T-1: “Christy application includes a first sketch and a reference to has recently made some estimates on the performance of notebook, A-147 . . . . The patent on the Christy Gadget a solid gadget.”(Fig. 11). was filed in Christy and Peierls’ names on 16 Jan. 1946. Both Christy and Peierls confirm that, although Peierls’ Subsequent 1944–1945 Los Alamos reports in the name is on the patent, the idea was Christy’s; Peierls was NSRC continue to refer to the Christy Gadget. The likely included since he was the T-1 group leader and did Gadget Physics (G) Division, led by Bacher, reported in calculations that showed feasibility. Peierls interviewed its first progress report, Oct. 1, 1944, that “an alternative by Hoddeson, 20–21 March 1986, OH-111, and Christy form of limited objective is now being considered interview by Hoddeson, 14 April 1986, OH-117.” quantitatively by Christy, viz a gadget that is integrated in compact solid sphere form with tamper and then We agree with the perspective that Peierls was included compressed by H.E.” On Oct. 30, in LA-164, Christy and because he did useful calculations. But we doubt that his Bethe presented calculations of a solid-sphere implosion position as T-1 group leader played a role since the as a limiting case of shell implosions, noting that the academic tradition at Los Alamos has always been such “compression will be finite even for a solid sphere.” that authorship is earned through substantive technical Experimental implosion work on compressions that could contributions. be achieved was reported in the Nov. 16, 1944, G Division progress report, LA-170, which stated that a 5. Open Sources “study of solid sphere (Christy gadget) is being done by the magnetic method (Fowler) and X-ray (Fowler, A. Hans Bethe Interview on YouTube Tuck . . .).” Hans Bethe was the leader of the Theoretical Division during the Manhattan Project and was intimately familiar D. Critical Assembly Book with the proposal and subsequent IBM calculations for Los Alamos historians gave a valuable perspective in the Christy Gadget. In a 1996 recorded set of interviews, 25 their remarkable 1993 book, Critical Assembly,16 but available on YouTube, Bethe said, “Christy unfortunately, only in its classified version. We are suggested . . . let the implosion act on a complete sphere making unclassified text from this source available to a of material; the implosion will compress it. . . . This broader readership. The book describes the patent and Christy method was adopted and was tested in what is states that the invention was really Christy’s. We quote called Alamogordo in the New Mexican desert.” As the 26 from the classified version of Critical Assembly, pp. 363– immediate leader and long-term friend of Peierls, 4, and its footnote 173: Bethe’s statement is compelling evidence for the role of Christy as the originator of the invention. “As Christy explained the discovery in the ‘Record of Invention’ for his 1946 patent application, ‘Since even B. Edward Teller Memoirs with these compromises, symmetry was still uncertain, it Edward Teller’s memoirs give sole credit for the idea of seemed desirable to examine theoretically the the Christy Gadget to Christy: 7 arrangement which was the limit of the thicker and thicker shells, and which seemed to be least subject to “Robert Christy pointed out that if we adopted a asymmetry, i.e. the solid gadget. It seemed clear that since simpler plan, we would eliminate the difficulties in a solid arrangement the active material starts off as [asymmetries resulting from an implosion of a hollow near the center as it can be placed, the motion of the sphere of plutonium]. Instead of imploding a thin shell of active material is minimized and if it moves little it should plutonium, we could implode an assembly of the high be unable to develop asymmetries as large as the actual explosives and tamper on a solid sphere of plutonium. motion.’ From estimates he made of the asymmetry, it With little movement of the plutonium, mixing would be appeared extremely likely that a solid gadget would not practically impossible. The calculations showed that the suffer from the major trouble of the hollow ones— namely converging power of the explosion, even in this asymmetry.173” conservative design, would produce enough compression for success. Footnote: That simple, ingenious suggestion “Footnote-173: Updated Christy document in patent made at an earlier time would have saved a great deal of files, Solid Gadget, A-84-015, 4-16,17,18; Christy effort. But the work was not wasted because it later helped interview by Hoddeson, 4 April 1986, OH-117, T- in producing improved designs.” (Ref. 7, p. 202)

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This text, including Teller’s footnote, makes it clear unknown. But Lorna Arnold’s claim that the solid sphere that the idea was not first proposed months earlier by design “was Peierls’ idea” can be refuted. There is no someone else; a paragraph later, he refers to this novel evidence we are aware of that supports her claim (this is idea as “Christy’s proposal.” Furthermore, Teller usefully something of a surprise given her many impressive illustrates how the proposal generated new confidence in accomplishments as an historian). Quite the opposite: in the implosion device. When James Conant, Harvard an oral history interview at Los Alamos, Peierls himself University and head of the National Defense Research said, “Now, Christy’s idea was to notice . . . That was a Committee, was informed of Christy’s new design, he very novel thought. Now, how likely it is that someone else muttered that “this is the first time I have really thought it would have come up with that, I just don’t know.” (See would work.” (Ref. 7, p. 203) Sec. 4B.) Peierls’ writings never suggested otherwise. The improved designs that Teller was referencing in his Los Alamos’ NSRC, along with numerous open source footnote were designs for levitated spheres and hollow statements, provide convincing evidence that Christy shells. The hollow-boosting shell concept was invented the idea of the solid sphere implosion design for successfully tested by Los Alamos in Operation Teapot the Fat Man weapon. The new information that we have (1955); Livermore subsequently made other useful design provided in Sec. 4 of this paper, presented as unclassified advances. extracts from the classified originals, is (a) the patent evidence showing the evolution of the invention patent C. Robert Christy Interview on YouTube from 1945 (in Christy’s name only) to 1946 (in Christy An interview with Christy is available on YouTube.27 and Peierls’ names); (b) our transcripts of the Los Alamos Again, Christy states his own role in inventing the gadget: 1986 oral history interviews of Christy and Peierls, see Sec. 4B; (c) the Theoretical Division monthly progress “I suggested that instead of having a hollow shell report by Bethe and Peierls, Sec. 4C; and (d) the which could implode in an irregular way, if you started perspectives documented in the classified version of the with an essentially solid ball and then hit that with an 1993 book Critical Assembly, Sec. 4D. explosive on the outside, that would be sure to stay like a In the classified version of Critical Assembly sphere and therefore it could not fail. . . . It would be (unclassified extracts reproduced in Sec. 4D), Hoddeson much more certain. This idea was bought as being the et al. suggest that Peierls’ name was added to the patent best way to proceed.” in 1946 because he conducted essential hydrodynamic calculations that showed it was feasible. We concur with Christy does not credit Peierls or anyone else as this view from the Los Alamos historians. This scenario inventor. reflects a common phenomenon in science—that while the idea might have originated with one person, the D. Juliana Christy’s Biography process of working out the idea, refining it, performing In Juliana Christy's hagiographic book about her late 28 the calculations, motivating key small-scale implosion husband, she quotes Robert Christy as stating: experiments, and proving it to be feasible was a collaborative effort between Christy and Peierls. We “The idea I had was to eliminate the hole in the middle speculate that after the war, Ralph Carlisle Smith was of the implosion bomb and make it a solid sphere that motivated to finalize and accurately clarify the many would be compressed by the high explosives.” patents. As a fair-minded attorney, he wanted to attribute E. ’ Book Dark Sun credit in the patent inventions as accurately as possible. Richard Rhodes should not be expected to have unique Furthermore, we noted that his letter shown in Fig. 6 insights into the questions explored in this paper. points to some input from government officials in Nevertheless, his books on the history of the fission and Washington DC to add Peierls to the patent, for reasons fusion bomb developments are well regarded and that can only now be speculated on. thorough. Surprisingly, the details of the solid core John von Neumann did, at an earlier time, suggest a invention are not given in The Making of the Atomic design with a levitated solid ball, but it was in a different Bomb. Instead, they appear in Dark Sun. There, Rhodes configuration than Christy’s idea (von Neumann’s had an credits only Robert Christy for proposing the use of a air gap). The argument that Teller originated the Christy solid-plutonium ball.29 Gadget has also been refuted: Teller himself emphasized his own original insight was in the increased compression 6. Conclusions in implosion, but he credits the “ingenious idea” of a solid The exact details of the creative events in the Theoretical plutonium sphere to Christy (Sec. 5B). Division during the fall of 1944 will likely remain forever Beyond the Christy gadget design, this paper has not addressed the origins of the implosion concept, except for

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mentioning the early interest by von Neumann, Teller, 7. Acknowledgment Tuck, and Neddermeyer. It is generally thought that in the We gratefully acknowledge useful discussions with Alan U.S. the idea goes back to Richard Tolman as noted in Carr, Madelaine Whitacre, Roger Meade, Lowell Wood, Serber’s Primer. Tom Kunkle is presently writing a paper Frank Close, Sabine Lee, Nic Lewis, Bill Archer, Craig on the German origins of implosion concepts, starting Carmer, and the AWE for their review, as well as very around 1940, and indeed whether any of this German valuable input from Peter Christy (Bob’s son). We also work was known by US Manhattan Project researchers at thank Dannie Alcazar for his assistance in locating the time as Lowell Wood has suggested30. documents in the NSRC, and Carl Gilbert for providing classification reviews. This is a Los Alamos unclassified The remarkable characters listed in our Dramatis document LA-UR-20-27638 version 3. Personae all played outsized roles in the development of the first atomic weapon. But as for the question of who was the originator of the Christy Gadget’s invention, it has been a case of much ado about nothing. It was Christy.

1 A. B. CARR, “Thirty Minutes Before the Dawn: The Story of Trinity,” 19 A. WELLERSTEIN, Christy’s Gadget: Reflections on a death,” WRL (this issue), American Nuclear Society Nuclear Technology http://blog.nuclearsecrecy.com/2012/10/05/christys-gadget/ (2012). (Submitted, 2021). 20 S. LEE, “‘In no sense vital and actually not even important’? 2 E. N. BROWN, D. L. BOROVINA, “The Trinity High Explosives Reality and Perception of Britain's Contribution to Implosion System: The Foundation for Precision Explosive the Development of Nuclear Weapons,” Contemporary British History, Applications,” WRL (this issue), American Nuclear Society Nuclear 20, 2, 159-185 (2006), DOI: 10.1080/13619460600600680. Technology (Submitted, 2021). 21 R. CHRISTY and R. PEIERLS, “Method and Apparatus for 3 A. B. CARR, “Documents Pertaining to the British Mission,” Los Explosively Releasing Nuclear Energy,” Los Alamos NSRC, 4-16, 17, Alamos report LA-UR-09-05504 (2009). R.C. Smith letter to Captain 18, S-3956-X; A-84-015. T.O. Jones, 18 September 1945. 22 T. A. CHADWICK and M.B. CHADWICK, ``Who Invented the 4 L. ARNOLD, Britain and the H-Bomb, Palgrave Macmillan, Trinity Nuclear Test’s Christy Gadget”, Los Alamos unclassified report Basingstoke, Hampshire, (2001). LA-UR-20-27638 version 2 (2020); Los Alamos WRL journal, Los 5 F. CLOSE, Trinity: The Treachery and Pursuit of the Most Dangerous Alamos report LA-CP-21-00045 (2021). Spy in History, Allan Lane (2019). 23 R. PEIERLS, “Oral History Interview with L. Hoddeson,” OH-111, 6 R. CHRISTY, Oral history interview with L. Hoddeson, April 14, audio tape 5 of 6, Los Alamos NSRC (March, 20-21, 1986). 1986, OH-117, Los Alamos NSRC. Audio file Robert_Christy_2of4, at 24 H. BETHE, “Theoretical Division Progress Report,” LAMS-149. 12–15 minutes in. (September, 1944). After Bethe’s introduction, each group leader 7 E. TELLER, Memoirs: A Twentieth-Century Journey in Science and provides a summary of their progress, including Peierls. Politics, Perseus Publishing, Cambridge, Massachusetts, 185 (2001). 25 H. BETHE, “Hans Bethe – Help from the British, and the ‘Christy 8 M. GOWING, Britain and Atomic Energy 1939–1945, Palgrave Gadget’ (94/158),” YouTube, Macmillan, Basingatoke, London, 389-394 (1964) See MAUD in https://www.youtube.com/watch?v=i7KwPpXr1zU. Appendix (p. 394) and Frisch-Peierls in Appendix (p. 389). 26 S. LEE, The Bethe-Peierls Correspondence, World Scientific, 9 J. R. OPPENHEIMER,“J.R. Oppenheimer letter to R. Peierls,” Los Singapore, (2007). Alamos document NSRC A-084-019 (1942). 27 R. CHRISTY, “Robert Christy – Constructing the Nagasaki Atomic 10 R. C. WILLIAMS and P. L. CANTELON, “Letters of J. Robert Bomb,” YouTube, Oppenheimer,” The American Atom, 36-37, University of Pennsylania https://www.youtube.com/watch?v=Ez45QEMI5CA&list=PLVV0r6C Press, . mEsFyeOKI21OdxEh1TgQlnJ_Uj&index=8. 11 B. ARCHER and N. MORGAN, ``On the origins of Lagrangian 28 I. J. CHRISTY, Achieving the Rare: Robert Christy Journey in hydrodynamic methods’’, this issue (ANS NT, submitted 2021) Physics and Beyond, World Scientific, Singapore, (2013). 12 A. B. CARR, “Documents Pertaining to the British Mission,” Los 29 R. RHODES, Dark Sun: The Making of the Hydrogen Bomb, Alamos Report LA-UR-09-05504 (2009). These letters, and many Touchstone, New York, (1996). others related to the British contributions, are available in a useful 30 L. WOOD, private communication to MBC, 9/1/2020. Lowell Wood, compilation by Alan Carr. a friend of Teller's, wrote: “According to the story-as-told-by-Teller, the 13 J. LESTONE, this issue (ANS NT, submitted 2021); J. LESTONE and essence of the Idea arose in a ‘casual’ discussion one evening at his C. BATES, “The Bethe-Feynman Formula and WWII Atomic Bombs,” ‘home’ on Bathtub Row, soon after the Lab opened, when a just-arrived- Los Alamos report LA-20-00318 (2020). in-town John von Neumann came over for dinner and the two of them 14 S. ULAM, “John von Neumann 1903–1957,” Bull. Amer. Math. Soc., fell to discussing fundamental alternatives to the ‘Little Boy’ concept, 64, 1-49 (1958). as a basic hedge against its then-already-well-understood-in-principle 15 R. MOORE, “Woolwich, Bruceton, Los Alamos: Monroe Jets and the failure modes. Von Neumann tabled the spherical implosion concept, Trinity Gadget,” This issue. American Nuclear Society Nuclear the outline of which he had heard from its seemingly-never-openly- Technology (Submitted, 2021). published German origins around 1940 – which concept however was 16 L. HODDESON, P. W. HENDRIKSEN, R. A. MEADE, and C. novel to Teller, who inquired for technical specifics, centered on then- WESTFALL, Critical Assembly, A Technical History of Los Alamos attainable velocities and certain hydrodynamic complexities. Von During the Oppenheimer Years 1943-1945, Cambridge University Press Neumann then estimated-on-the-spot the quantitative aspects of interest (2008). The classified version has reference number CRM-HP-88-34 to Teller, who apparently observed on-the-spot that then-remarkably and is available on the ALDX Online Vault as BC01126607AN. high pressures would seemingly be realized at implosion culmination, 17 R. C. SMITH, “British Mission,” Los Alamos memorandum LAB- which he commented were large compared to those at Earth’s center ADCS-127 (1949). Included in Ref. 12. (due to his earlier geophysics interests). The two of them then rather 18 F. M. SZASZ, British Scientists and the Manhattan Project, simultaneously realized that substantially-greater than zero-pressure Macmillan Academic and Professional LDT (1992). solid-densities might thereby be realized in the vicinity of implosion

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and the story-from-there may be found in the set of LAMS documents bearing the names of one-or-more of Neddermeyer, Teller and von centers, which they proceeded to estimate – and to make seemingly- Neumann – as well as many others. obvious inferences re. the ‘impacts’ on the critical masses of fast-fissile (Yes, I’m aware that this narrative is notably different from the implosion-processed materials-of-interest. The following day, Teller long-since-Official Story – but it’s the best that I have on-offer, and no related, he and von Neumann took the concept to Oppenheimer, who one ``inside'' ever controverted its essentials in my hearing)." (2020). rather immediately directed the commencement of high-priority effort, for which effort was conscripted by Oppenheimer –

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Figure 1. Robert Christy photographed soon after his arrival in Los Figure 3. John von Neumann. Alamos.

Figure 2. Rudolf Peierls. Figure 4. Ralph Carlisle Smith.

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Figure 5. An image of the Theoreotical Division Organizational Chart during the Manhattan Project, May 10 1945. There were 18 researchers at Los Alamos during the Manhattan Project who either had, or would be later given, Nobel Prizes. T-Division had four of them (Bethe, Glauber, Reines and Feynman).

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Figure 6. An image of another extract from the original March 6, 1945, patent with Robert Christy as the sole author. This extract references other related, but different, inventions patents by Neddermeyer, von Neumann, and Serduke that are also held in Los Alamos’ NSRC, see Sec. 3. It also details the Proposed Claim. In Fig. 7 we show an example of the claim in the final Christy-Peierls patent, that is very similar.

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It is claimed:

Figure 7. This image is from the (now joint) Christy-Peierls patent from January 1946, showing items 28 and 29, which follow “It is claimed:” There are 28 such paragraphs, all involving just small variations of the same claim. An earlier similar version of the claim is shown in Fig. 6, for the Christy-only patent.

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Figure 8. An image of an extract from the original March 6, 1945, patent with Robert Christy as the sole author/inventor, showing the Record of Invention. It is signed by Ralph Carlisle Smith, the person in charge of patent applications at Los Alamos. Note that it refers to the conception as “prior to 2 Oct. 1944” and references the first sketch or drawing in Christy's laboratory “Notebook A-147.” Bethe's Theoretical Division monthly progress report LAMS-149 from September 1944 also references Christy's idea (see Fig. 11). Note the typo in the title (“neutron energy” instead of “nuclear energy,” not present in the original handwritten version in Fig. 9.

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Figure 9. The upper image is from the early 1945 Christy-only authored patent. The lower image comes from the point at which Peierls was added as a co-inventor (Peierls added his name as an author, by hand).

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Figure 10. This image shows Ralph Carlisle Smith’s February 1946 letter to Christy, where a decision has been made to add Peierls’ name as co-inventor to the patent. 18 Submission for ANS NT issue, LA-UR-20-27638 version 3

Figure 11. Images from the September 1944 Theoretical Division Progress Report by Hans Bethe, document LAMS-149. These remarkably informative monthly progress reports follow a standard format: Bethe provides an introduction, and then each group leader provides a summary of their group’s work. These extracts show the first Los Alamos report documentation of Christy’s invention of his gadget idea, in Bethe’s and Peierls’ words.

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